1. Field of the Invention
The present invention relates to a tool to be attached to a spindle of a machine tool for machining a workpiece.
2. Description of Related Art
In for example a machining center or other machine tool provided with a spindle, the maximum rotational speed of the spindle is determined by the structure of a main bearing rotatably supporting the spindle and a lubrication system of this bearing. For this reason, when it is necessary to rotate a tool at a higher rotational speed than the maximum rotational speed of the spindle, an accelerating apparatus is used.
As the accelerating apparatus, for example, an accelerating apparatus provided with a gear mechanism such as epicyclic gearing which holds the tool and is removably attachable to the spindle is well known.
For example, in a machining center, when it is desired to increase the rotational speed of the tool to higher than the maximum speed of the spindle temporarily, an accelerating apparatus such as the above accelerating apparatus is attached to the spindle in the same way as an ordinary tool to enable the tool to be rotated at a higher rotational speed.
However, when raising the rotational speed of the tool to a higher speed than the maximum rotational speed of the spindle by the above gear mechanism, the accelerating apparatus increasingly generates heat at a super high rotational speed such as tens of thousands to hundreds of thousands of revolutions per minute, so the machining tolerance of a workpiece can be influenced by the heat. Further, at the above super high rotational speed, the noise from the accelerating apparatus can also increase. Furthermore, a highly reliable precision structure able to withstand the above super high rotational speed is required for the accelerating apparatus, so there is the disadvantage that the manufacturing cost becomes relatively high.
Further, in a case of an accelerating apparatus with a gear mechanism, it is needed to lubricate the gear or bearing and arrange a supply passage and a discharge passage for the lubricating oil in the accelerating apparatus, so there is the disadvantage that the apparatus becomes larger and it is difficult to automatically change the tool by an automatic tool changer.